Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving surface to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being for example 300 dpi, 600 dpi, and 1200 dpi.
Ink jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving substrate. In an offset printing system, the image is formed on an intermediate transfer surface and subsequently transferred to the final receiving substrate. The intermediate transfer surface may take the form of a liquid layer that is applied to a support surface, such as a drum. The print head jets the ink onto the intermediate transfer surface to form an ink image thereon. Once the ink image has been fully deposited, the final receiving substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred to the final receiving substrate.
Ink jet printing systems using intermediate transfer, transfix or transfuse members are well known, such as those described in U.S. Pat. Nos. 4,538,156, 6,843,559, and 6,196,675, the disclosures of each of which are hereby each totally incorporated by reference herein in their entireties.
Generally, the intermediate transfer, transfix, or transfuse member (collectively referred to as intermediate transfer member hereinafter for simplicity) is employed in combination with a printhead. A final receiving surface or print medium is brought into contact with the imaging surface after the image has been placed thereon by the nozzles of the printhead. The image is then transferred and fixed to a final receiving surface.
More specifically, the phase change ink printing process begins by first applying a thin liquid, such as, for example, silicone oil, to an intermediate transfer member surface. The solid or hot melt ink is placed into a heated reservoir where it is maintained in a liquid state. This highly engineered ink is formulated to meet a number of constraints, including low viscosity at jetting temperatures, specific visco-elastic properties at component-to-media transfer temperatures, and high durability at room temperatures. Once within the printhead, the liquid ink flows through manifolds to be ejected from microscopic orifices such as through use of piezoelectric transducer (PZT) printhead orifices. The duration and amplitude of the electrical pulse applied to the PZT is controlled so that a repeatable and precise pressure pulse can be applied to the ink, resulting in the proper volume, velocity and trajectory of the droplet. Several rows of jets, for example four rows, can be used each one with a different color. The individual droplets of ink are jetted onto the liquid layer on the intermediate transfer member. The intermediate transfer member and liquid layer can be, if desired, held at a specified temperature such that the ink hardens to a ductile viscoelastic state.
After depositing the image, a print medium can, if desired, be heated by feeding it through a preheater and into a nip formed between the intermediate transfer member and a pressure member, either or both of which can also be heated. A hard synthetic pressure member is placed against the intermediate transfer member in order to develop a high-pressure nip. As the intermediate transfer member rotates, the heated print medium is pulled through the nip and is pressed against the deposited ink image with the help of a pressure member, thereby transferring the ink to the print medium. The pressure member compresses the print medium and ink together, spreads the ink droplets, and fuses the ink droplets to the print medium. Heat from the preheated print medium heats the ink in the nip, making the ink sufficiently soft and tacky to adhere to the print medium. When the print medium leaves the nip, stripper fingers or other like members, peel it from the intermediate transfer member and direct it into a media exit path.
To optimize image resolution, it is desirable that the transferred ink drops spread out to cover a predetermined area, but not so much that image resolution is compromised or lost. Finally, image transfer conditions desirably are such that nearly all the ink drops are transferred from the intermediate transfer member to the print medium. Therefore, it is desirable that the intermediate transfer member has the ability to transfer the image to the media sufficiently.
The intermediate transfer member can be multi-functional. First, the ink jet printhead prints images on the intermediate transfer member, and thus, it is a print medium. Second, after the images are printed on the intermediate transfer member, they can then be transfixed or transfused to a final print medium. Therefore, the intermediate transfer member provides a transfix or transfuse function, in addition to an image receiving function.
In order to ensure proper transfer and fusing of the ink off the intermediate transfer member to the print medium, certain nip temperature, pressure and compliance are selected. Unlike laser printer imaging technology in which solid fills are produced by sheets of toner, the solid ink is placed on the intermediate transfer member one pixel at a time and the individual pixels spread out during the transfix process to achieve a uniform solid fill. Also, the secondary color pixels on the intermediate transfer member are physically taller than the primary color pixels because the secondary pixels are produced from two primary pixels. Therefore, compliance in the nip enables conformity around the secondary pixels and allows the primary pixel neighbors to touch the final print medium with enough pressure to spread and transfer. The correct amount of temperature, pressure and compliance is selected to produce acceptable image quality.
U.S. Pat. No. 5,389,958 entitled “Imaging Process” which is hereby totally incorporated by reference herein in its entirety, is an example of an indirect or offset printing architecture that utilizes phase change ink. The intermediate transfer surface is applied by a wicking pad that is housed within an applicator apparatus. Prior to imaging, the applicator is raised into contact with the rotating drum to apply or replenish the liquid intermediate transfer surface.
Once the liquid intermediate transfer surface has been applied, the applicator is retracted and the print head ejects drops of ink to form the ink image on the liquid intermediate transfer surface. The ink is applied in molten form, having been melted from its solid state form. The ink image solidifies on the liquid intermediate transfer surface by cooling to a malleable solid intermediate state as the drum continues to rotate. When the imaging has been completed, a transfer roller is moved into contact with the drum to form a pressurized transfer nip between the roller and the curved surface of the intermediate transfer surface/drum. A final receiving substrate, such as a sheet of medium, is then fed into the transfer nip and the ink image is transferred to the final receiving substrate.
To provide acceptable image transfer and final image quality, an appropriate combination of pressure and temperature is applied to the ink image on the final receiving substrate. Reference, for example, U.S. Pat. No. 5,777,650 entitled “Pressure Roller” which is hereby incorporated by reference herein in its entirety, which discloses a roller for fixing an ink image on a final receiving substrate.
In a color printing system, the ink image on the final receiving surface is composed of individual drops of ink that form primary and secondary colors. The primary and/or secondary colors may include two or more drops of ink placed on top of one another. In the image transfer process, the ink image is transferred from the intermediate transfer member to the final receiving substrate. A portion of the ink image is fused or pressed into the final receiving substrate. The height of the remaining ink that lies above the surface of the final receiving substrate is referred to as the “ink pile height.”
Piezoelectric ink jetting (PIJ) can be made by building a print image on an intermediate transfer member. The viscosity of the image desirably is greatly increased after jetting to obtain a stable, transfusable, and fusable image. Phase change inks can be used in this architecture. Another option for this type of architecture comprises printing directly onto paper.
The disclosures of each of the foregoing U.S. Patents and applications are hereby incorporated by reference herein in their entireties. The appropriate components and process aspects of the each of the foregoing U.S. Patents and applications may be selected for the present compositions and processes in embodiments thereof.